Large commercial washing machines are in wide use in a variety of commercial, industrial and institutional settings. These machines typically use chemicals such as detergent, softener, sour (i.e. acid for maintaining a proper pH level in the wash water) and a chlorine-based bleach such as sodium hypochlorite. In the normal functioning of the machine, a sensor-and-timer-driven microprocessor typically controls a group of separate pumps which add these chemicals to the wash water at the proper time and in the proper quantities for optimum washing action.
One of the functions of the microprocessor is to prevent the addition of sour and bleach to the water simultaneously, i.e. before one of these components has become sufficiently diluted to prevent an adverse reaction with the other. Because the undiluted mixing of sour and bleach causes the release of toxic chlorine gas, the proper functioning of the microprocessor and pump circuitry in this respect is vital.
A number of solutions have been proposed to guarantee the reliability of the dispensing control. For example, it has been proposed to provide the microprocessor with redundant or self-checking circuitry; to provide sensors to shut down the pumps if chlorine gas is detected; and to introduce the chemicals into the water at different locations. None of these approaches have, however, been satisfactory and economically practical.
The fact remains that with many such machines in use, the eventual failure of the pump control circuitry in one of them is a statistical near-certainty. Indeed, such failures with potentially catastrophic consequences have already occurred. Consequently, a need exists in the commercial washing machine industry for an inexpensive fail-safe device which can be incorporated into the controller or easily installed as an aftermarket device, and which makes it physically impossible for the sour and bleach pumps to be energized at the same time.